Sustainable Synthesis of Nitrogenated Compounds

Chemical manufacturing, a major contributor to greenhouse gas emissions, must undergo decarbonization to achieve global net zero emissions by the end of 21st century. The synthesis of NH3 via the energy intensive Haber-Bosch process is a significant contributor to greenhouse gas emissions. Electrochemical NH3 synthesis offers a decentralized and green alternative way which can be achieved by two approaches, namely the direct N2 reduction by using water as the proton source and the mediator approach by using Li to indirectly activate N2. Apart from being an energy intensive process, the Haber Bosch process is also responsible for the disruption of the N2 cycle by releasing reactive nitrogen into the environment. NO3- is a significant form of the reactive nitrogen found in agricultural run-off water, industrial effluents, and ammunition waste. This thesis focusses on two key areas: the electrochemical valorization of waste NO3- and the electrochemical reduction of N2 into NH3. Catalyst screening is performed for the selective electrochemical synthesis of NH3 and urea from NO3-. Co is the active catalyst for synthesizing NH3 and Ag is the active catalyst for producing urea. A maximum NH3 FE of 92.37 ± 6.7 % and an NH3 current density (CD) of 565.26 ± 23.56 mA/cm2 is obtained at -0.8 V vs. RHE on the oxide derived Co, and the process can be operated transiently based on the availability of renewable energy for varied NO¬3- concentrations without loss of selectivity. A maximum urea CD of ~-100 mA/cm2 and a urea FE of ~100% is observed at -1.25 V vs. RHE in an Ag gas diffusion electrode (GDE) configuration. A rational approach is developed to design catalysts, electrolytes, and an electrochemical cell for the electrochemical reduction of N2 to NH3 in aqueous media. The effect of operating conditions such as pressure, ethanol concentration, and Li salts are investigated for the Li mediated NH3 synthesis process. A maximum NH3 FE of ~70 % and a maximum NH3 CD of ~-100 mA/cm2 are observed at an applied current density of -150 mA/cm2, when 0.065 ethanol concentration, 20 bar N2 pressure, and 3 M LiBF4 are used.